Various radical polymerization processes exist.
Mention may be made first of methods that use organic solvents such as secondary alcohols, for instance isopropanol. These methods are nowadays unsatisfactory since they generate volatile organic compounds (VOC).
Firstly, it is necessary to remove these solvents at the end of reaction, which has the effect of complicating the industrial process for preparing the polymer.
Secondly, the effects on health and on the environment of these solvents are known to be very detrimental, and as such it is sought to avoid producing any. Finally, even after purification (distillation), there are still traces of solvent in the polymer solution.
Other methods exist for synthesizing polyacrylic polymers, which take place in water and do not generate volatile organic compounds.
Among the various radical polymerization processes, mention may also be made of the controlled radical polymerization of RAFT type (reversible addition fragmentation chain transfer) which makes it possible to perform the living polymerization of a monomer. Such a process also makes it possible to obtain polymers that have low polydispersity indices PI (also known as the polymolecularity index), which makes them particularly efficient for certain applications.
To perform a controlled radical polymerization of RAFT type, and thus to obtain a polymer of expected molecular mass which has a good PI index, it is important to introduce into the reaction medium an available amount of chain-transfer agent, in other words to use an amount of chain-transfer agent such that each chain to be polymerized is functionalized with a chain-transfer agent. In addition, it is important for this chain-transfer agent to be available from the outset when the polymerization is initiated, i.e. when the polymerization reactor is heated and radicals are generated. This implies that large amounts of chain-transfer agent must be used in a controlled radical polymerization process of RAFT type.
Despite all the advantages resulting from a RAFT polymerization, the use of such amounts of chain-transfer agent present a certain number of drawbacks.
Firstly, it turns out that chain-transfer agents are expensive products, which has an appreciable impact on the cost of the polymer obtained.
Furthermore, when sulfur-containing chain-transfer agents as described in documents WO 02/070571, WO 2005/095466 and WO 2006/024706 are used, it is found that a fraction of these compounds will be degraded into free sulfur-containing byproducts such as CS2 and H2S, and will be found in the final aqueous polymer solution and in the process drain waters, which may thus have a negative impact on man and on the environment. In addition, the presence of these sulfur-containing byproducts in the aqueous solution gives rise to the evolution of gases that are harmful to humans when the polymer is used.
Alternative methods to controlled radical polymerization of RAFT type exist. According to one of these methods, hydrogen peroxide is used, which acts as initiator, along with, for example, copper sulfate which acts as catalyst and chain-transfer agent. However, to arrive at a polymer which has a molecular mass of less than 8000 g/mol, for example about 6000 g/mol, it is necessary to use large amounts of catalyst, which gives rise to large amounts of pollutant byproducts.
Alternatively, thiolactic acid is used, or another mercaptan RSH, as additional chain-transfer agent, but, once again to obtain a polymer which has a molecular mass of less than 8000 g/mol, for example about 6000 g/mol, large amounts of thiolactic acid or, more generally, of transfer agent need to be used. Yet other processes make use of sodium-hypophosphite, of chemical formula NaPO2H2, as chain-transfer agent and redox agent, in the presence of hydrogen peroxide or of a radical generator. GB 771 573 A1 especially describes such a process. This has the major drawback of requiring large amounts of sodium hypophosphite, one fraction of the phosphorus becoming grafted into the polymer, another fraction of the phosphorus being found in the form of phosphorus salts in the process waters. This constitutes, firstly, a drawback when the polymer is used and, secondly, a pollutant for the environment.